This invention relates to zeolite bound zeolites which have enhanced activity, selectivity, and/or activity maintenance when utilized in hydrocarbon conversion processes.
Zeolite materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, zeolite materials have been used as adsorbents, catalyst carriers for various types of hydrocarbon conversion processes, and other applications. Zeolites are complex crystalline aluminosilicates which form a network of AlO.sub.4 and SiO.sub.4 tetrahedra linked by shared oxygen atoms. The negativity of the tetrahedra is balanced by the inclusion of cations such as alkali or alkaline earth metal ions. In the manufacture some zeolites, non-metallic cations, such as tetramethylammonium (TMA) or tetrapropylammonium (TPA), are present in the synthesis. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes and catalysts for chemical reactions and catalysts carriers in a wide variety of hydrocarbon conversion processes.
Zeolites include materials containing silica and optionally alumina, and materials in which the silica and alumina portions have been replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Unless otherwise specified, the terms "zeolite" and "zeolite material" as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum.
Synthetic zeolites are usually prepared by crystallization from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because the powder has no significant mechanical strength.
Mechanical strength may be conferred on a zeolite by forming a zeolite aggregate such as a pill, sphere, or extrudate. An extrudate can be formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. Examples of such binders include materials such as alumina, silica, titanium, and various types of clays.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when the bound zeolite is used in a catalytic conversion process, the activity, selectivity, activity maintenance, or combinations thereof of the catalyst can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 60 wt % of zeolite, the binder dilutes the adsorptive properties of the zeolite. In addition, since the bound zeolite is prepared by extruding the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite, otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion processes and other applications. Still further, when a bound zeolite is used in catalytic processes, the binder may affect the chemical reactions that are taking place within the zeolite and also may catalyze undesirable reactions which can result in the formation of undesirable products.
Thus, there is a need for a hydrocarbon conversion process which utilizes a zeolite catalyst which overcomes, or at least mitigates the above described problems.